The present invention relates to measurement of a position using radio signals, and in particular, to a technique suitable for the position measurement using a cellular base station as a signal source.
For a position measuring technique of a terminal using broadband radio signals, U.S. patent application Ser. No. 09/791,862 filed on Feb. 26, 2001 describes a system using code division multiple access (CDMA) signals.
FIG. 12 shows a concept of the system. A terminal 1201 receives signals (transmitting codes) 1206, 1207, and 1208 respectively sent from base stations 1202, 1203, and 1204 at timing synchronized according to a global positioning system (GPS) 1205 or the like to create delay profiles indicating reception timing of the signals. The terminal 1201 estimates signal propagation distance using the delay profiles and then the terminal position according to positions of known base stations and the signal propagation distance.
FIG. 13 shows an example of the delay profile. The profile is obtained by calculating a correlation between the transmitted signals (codes) and received signals. A point of time (timing) of a high correlation indicates reception timing of the signals. Therefore, the profile is used to estimate propagation time.
Geometrically, if propagation distance can be estimated for each of signals received by a terminal from three base stations at mutually different positions, a position of the terminal can be estimated according to a principle of trilateration. If propagation distance can be estimated for each of four or more base stations, the terminal position can be estimated with higher precision using, for example, a method of least squares.
Therefore, to determine a position of the terminal, it is desirable to create delay profiles of signals or codes transmitted from at least three base stations at mutually different positions. To improve precision of the estimated terminal position, the number of base stations at mutually different positions is desirably increased. However, to create a delay profile for a transmitting or transmission code, a large number of steps and a large amount of memory are required. That is, for the creation of delay profiles, the number of transmitting codes from base stations is limited by size of the circuit to determine the terminal position. The number of transmitting codes is more strictly limited if it is desired to reduce the terminal size and consumption power.
Each of base stations available as signal sources uses directional antennas and hence forms a plurality of cells each of which has a form of a sector and is hence called a sector. Such a configuration of base stations is ordinarily used for, for example, wireless base stations of mobile telephones or cellular phones.
FIG. 14 shows an example of the configuration. A base station 1410 uses directional antennas 1401, 1402, and 1403 to form sectors 1411, 1412, and 1413, respectively. Since mutually different signals (transmitting codes) are transmitted in the respective sectors, the base station has a plurality of signal sources emitting signals in mutually different directions.